The third year of this study of the properties of the Field Ionization Scanning Ion Microscope (SIM) will be devoted primarily to three areas. Firt, the problem of specimen damage will be further investigated by irradiating organic specimens with different species of ion beams (H2 ion plus H ion, He ion, Ar ion). The specimens will be sectioned with an ultramicrotome so that regions of the specimen beyond the range of the ion beams can be compared with those traversed by the beams. The specimens will be examined with a transmission electron microscope. Next, it is planned to take micrographs of hard and soft tissue specimens with the SIM and then to compare these with micrographs of the same specimens taken with a scanning electron microscope. This will permit a study of the difference in contrast mechanisms between the two types of beams. The third area will be an investigation of the limit of resolution of the SIM. This will involve the use of He rather than H2 as the source gas to reduce the chromatic spread of the beam and the use of objective apertures in the .003-.005 mr range rather than the present .009-.010 mr range. The current-resolution characteristics can then be tested and compared with the theoretical predictions, which have already been confirmed for H2.